In storage systems, and storage controller and operating system file systems, there is a known snapshot technique for use in data backup or similar where a copy (snapshot) of a volume that is in a resting state is created at a specified point in time.
There is also a known differential snapshot technique that acquires a snapshot of lower capacity than methods that create a physical copy of an entire LU or journal methods that record all data updates in chronological order, by copying, when there is any difference between the snapshot and the logical unit (LU) where the most recent data exists—the production LU, only the data from the area where the difference is.
As differential snapshot techniques, preserving snapshot data by copying pre-update data (backward differential data) to another area (Copy-on-Write) when a LU is updated after acquisition of a snapshot—the Copy-on-Write method (U.S. Pat. No. 5,649,152); and preserving snapshot data by not overwriting the original data, but writing update data (forward differential data) to another area when a LU is updated—the Redirect-on-Write method (U.S. Pat. No. 5,819,292 and U.S. Pat. No. 7,174,352) are known.